The present invention relates to a fluorine-containing polymer solid electrolyte which comprises a non-crystalline fluorine-containing polymer having nonionic functional group and a side chain having structural units derived from a specific fluorine-containing ether, an electrolytic compound and further a solvent as case demands.
The fluorine-containing polymer solid electrolyte is useful as an electrolyte for electrochemical devices, particularly various batteries, capacitors, solid display devices and sensors.
So far ion conductors in the form of solution or paste have been used as an electrolyte for electrochemical devices such as batteries, capacitors and sensors from the viewpoint of good ion-conducting property. However electrolytes of such forms have problems with safety such as damage of a device due to leakage of solution and occurrence of fire. Also it is pointed out that there are problems that since scattering of solution arises at a sealing part of a housing case of a device, a high technique for sealing is required, and further a separator for impregnation of an electrolytic solution is necessary and therefore there is a limit in downsizing of devices and making devices thin.
On the other hand, there are proposed solid electrolytes of inorganic crystalline substances, inorganic glass, organic polymeric substances and the like. Representative examples of inorganic materials are ceramics such as β-Al2O3. However since those ceramics exhibit ion-conducting property only at high temperatures of not less than 300° C., they cannot be applied to batteries for use at ordinary temperature, and applications thereof are limited. Also ceramics are very fragile and have disadvantages that breakage occurs when assembling or using devices and molding thereof is difficult. Therefore ceramics are not practicable.
Organic polymeric substances are generally excellent in processability and moldability, and obtained solid electrolytes have flexibility and processability in bending. Therefore, a degree of freedom in device design is increased, and development thereof is expected. At present, there is available a complex of polyethylene oxide (PEO) and inorganic salt as a representative organic solid polymer having ion-conducting property, which however, has low ion-conducting property when an electrolytic solution is not contained therein, and is not practicable. Therefore an amorphous polypropylene oxide (PPO) and a PEO derivative having an oligoethylene oxide chain in its side chain are studied, but ion-conducting property thereof is still insufficient (for example, JP8-113496A, JP8-173435A, JP8-183186A, etc.).
Also there is a trial of enhancing ion-conducting property by adding, to a PEO/inorganic salt complex, a proper amount of plasticizer such as an organic solvent. However since the mixture has flowability and cannot maintain a solid state, there are disadvantages that assembling work of device becomes difficult, fire arises more easily and strength is low.
Heretofore polymers having anionic functional group such as —SO3− or —COOC− at an end of a side chain thereof have been primarily studied as an ion-conducting material of fluorine-containing polymeric substance. Those fluorine-containing polymers act as a polymeric anion and since the polymers function as a single ion conductor for conduction of a cation only, application thereof is limited and applications mainly to an electrolyte for fuel cells and ion exchange membrane have been studied (for example, U.S. Pat. No. 3,282,875, Journal of Power Sources, No. 29, p. 399 (1990), Journal of Electrochemical Society, No. 135, p. 2209 (1988), etc.).
With respect to an ion-conducting material which is prepared from a fluorine-containing polymeric substance having no ionic functional group, there have been widely studied, as a gel electrolyte for lithium battery, a compound comprising polyvinylidene fluoride and electrolytic solution and a compound comprising a polyvinylidene fluoride-hexafluoropropene copolymer and electrolytic solution (for example, Polymer Bulletin, Vol. 7, pp. 271-275 (1982), U.S. Pat. No. 5,296,318, etc.). A polymer battery obtained from polyvinylidene fluoride is electrochemically stable and has a characteristic that the polymer itself is difficult to burn since fluorine atom is contained therein, but has a disadvantage that when temperature is raised, the electrolytic solution oozes from the polymer. For the purpose to improve an ability of holding an electrolytic solution, a polyvinylidene fluoride-hexafluoropropene copolymer is used, but since the polymer itself has no ion-conducting property, it cannot be used for a solid electrolyte, and also even when it is used as a gel electrolyte, a large amount of electrolytic solution need be contained in order to obtain sufficient ion-conducting property. As a result, there remain problems with safety such as damage of device due to leakage of solution and occurrence of fire, and a high technique for sealing is required to prevent scattering of solution at a sealing part of a housing case of a device.
On the other hand, a PPO derivative prepared by converting methyl group of PPO to trifluoromethyl has been studied mainly as an electrolyte for lithium battery (for example, JP6-103764A, JP8-217872A, JP8-222270A, JP9-48832A, etc.). However there are disadvantages that when an electrolytic solution is not contained, ion-conducting property is low, oxidation resistance and heat resistance are not sufficient and a mechanical strength is low as well as a disadvantage that the PPO derivative subjected to crosslinking with an acrylic crosslinking agent is electrochemically unstable.